1. Field of the Invention
The present invention relates to a method for forming an electrophotographic image and electrophotographic device and, more particularly, to a method for forming an electrophotographic image and electrophotographic device using a toner having a total surface area ratio of additive in toner of 0.5-1.5 and an angle of repose of toner to the protective layer of a electrophotographic photoconductor of 30° or less.
2. Description of the Related Art
Conventionally, there are various electrophotographic methods known, which generally relates to a method of forming an electrostatic latent image on an image carrier (photoconductor) by various means utilizing a photoconductive material, developing the latent image with toner to form a visible image, transferring the toner image to a transfer material such as paper if necessary, and fixing the toner image on the transfer material by applying heat, pressure or the like to create a printed article.
For known methods for visualizing the electrical latent image, cascade development method, magnetic brush development method, pressure development method and the like may be mentioned. Further, there is also known a method in which magnetic toner released by a rotating sleeve having magnetic pole in the center is fed into an electric field created in between a photoconductor and a sleeve.
A one-component developing method allows reduction in size and weight of a developing device itself because it does not require carrier particles such as glass bead, iron powder, or the like as required in a two-component method. In order to maintain the toner concentration in a carrier constant, the two-component development method requires a device for detecting the toner concentration and supplying necessary amount of the toner, hence the development device is increased both in size and weight. On the other hand, one-component developing method is preferable in terms of reduction in the size and weight of the developing device because a device for detecting is not necessary.
For printer devices, LED and LBP printers are becoming mainstream technology in the market accompanying a technical trend to attain higher resolution, namely 400, 600 dpi in the past to 800, 1200 dpi at present. Concurrent with this trend, demand for higher definition in the development method is also pursued. Further, in the field of copying machines, digitalization prevails to cope with demand for improved functionality. Since the digitalization primarily intends formation of an electrostatic image with laser, aiming at higher resolution is becoming the focus of advancement of the technical trend. Accordingly, the developing method also calls for higher resolution/higher definition in the field of copying machines similarly to printers. Therefore, particle diameter of a toner is increasingly becoming smaller, and for instance, toners having small particle diameter existing in specific particle diameter distributions are proposed in Japanese Patent Application Laid-Open Nos. 3-181952, 4-162048, and the like.
The toner image formed on a photoconductor during developing process is transferred to a transfer material in the transfer process, and the residual toner untransferred on the photoconductor is cleaned in the cleaning process and stored in a waste toner vessel. In the cleaning process, generally, blade cleaning, fur brush cleaning, roller cleaning and the like are used. From the viewpoint of device configuration, such cleaning device is an inevitable equipment, and thus leads to an enlargement of the device as a whole, causing difficulty in realizing a device compact in size. Further, from the viewpoint of environmental protection, a system which minimizes the toner waste and promoting effective use of toner is desired, along with toner having high transfer ability.
According to the reduction in the size (or diameter) of toner particle, the adhesive force of the toner particle to a latent image carrier (mirror image force, van der Waals force, etc.) tends to increase, compared to the Coulomb's force applied to the toner particle at the time of transfer, consequently causing amount of the toners untransferred to increase.
In a roller charging method, the physical and chemical effect of the electrostatic latent image carrier surface by a discharge generated between an charge roller and the electrostatic latent image carrier is high, compared with that in a corona electric charging method, and a wear caused by the deterioration of the surface of photoconductor tends to occur, particularly, when an organic photoconductor/blade cleaning are used in combination, thus leaving a problem of shortened life (the combination of direct charging/organic photoconductor/one-component magnetic development method/contact transfer/blade cleaning is the mainstream technology in copying machines, printers, facsimiles and the like in the field demanding low price, small size and light weight, because reduction in cost, size and weight of an image forming device is relatively easy).
A study on adding filler to the protective layer of the electrophotographic photoconductor was carried out in an attempt to prevent the wear of the electrophotographic photoconductor (improvement in printing resistance). A study on protective layer used as surface layer of the photoconductor initially directed on organic photoconductors, including, for example, those disclosed in Japanese Patent Publication Nos. 2-3171, 2-7058, 3-43618, and the like. When the protective layer is provided on the surface of an inorganic photoconductor, fillers having relatively low resistance were suitably used as the protective layer (Japanese Patent Application Laid-Open Nos. 63-254462 and 63-254463). Therefore, electricity was charged more often in the protective layer as a whole or in the interface of protective layer/inorganic photosensitive layer rather than on the surface of the electrophotographic photoconductor. When the latent image was formed not on the surface of electrophotographic photoconductor but on the inner portion of the protective layer (including the interface with the inorganic photosensitive layer), an advantage was confirmed in that the influence of the shapes (flaw, etc.) which appear on the surface of the electrophotographic photoconductor was minimized. However, in order to render the surface layer to act as the protective layer, it is necessary to add a large amount of a conductive metal oxide as the filler to be added to the surface layer. In this case, even if the transparency of the surface layer is ensured by choosing an appropriate material, the bulk or surface resistance of the surface layer deteriorates, often causing image blurring in repeated use. To solve such disadvantages, Japanese Patent Publication No. 2-7057 and Patent No. 2675035 discloses a method to change the conductive metal oxide concentration in the surface layer in the depth direction of the coating surface, whereby the image blurring and flowing are suppressed.
In order to suppress image blurring, a method for mounting a drum heater to heat the electrophotographic photoconductor during the process is disclosed. However, in order to mount the drum heater to prevent image blurring by heating the electrophotographic photoconductor, the electrophotographic photoconductor needs to have large diameter, and therefore such method could not be applied to electrophotographic photoconductors having small diameters which are the focus of mainstream technology accompanying miniaturization of electrophotographic devices. Moreover, it is difficult to improve durability of the electrophotographic photoconductors having minor diameters. Further, the size of the device is inevitably increased by mounting the drum heater, thus causing increase in electric power consumption, and time consuming start up and the like, leaving various problems unsolved.
On the other hand, a surface layer (protective layer) using a filler having low resistance was laminated on a electrophotographic photoconductor using an organic charge generating material and charge transporting material (referred to as OPC) using the technique above-mentioned, and tested in repeated uses. As a result, image flowing was observed assumedly due to the poor matching property with OPC. Substantially the same result was observed in a method creating a concentration distribution in the surface layer of the conductive metal oxide, the method which was effective for inorganic photoconductors. The reason for the cause of image flow is not clear. In the recent electrophotographic process using organic photoconductors, a digital signal is used in a manner to be dotted-in when writing on the photoconductor, the manner which is very different from the manner applied for inorganic photoconductors. From the viewpoint of machine configuration, the level of resolution required has changed dramatically, thus rendering such phenomenon (defects) obvious.
Under such circumstances, it is essential to use a non-conductive and highly resistant filler in the surface layer of the organic photoconductor. However, use of highly resistant filler often causes a problem of increased residual potential. The frequently observed increase in residual potential leads to a high bright part potential within the electrophotographic device, causing reduction in image density or tones. Although it is necessary to raise the dark part potential for compensation, the increase in dark part potential brings up the field intensity, which not only causes an image defect such as toner deposition on the background of images, and the like but also leads to reduced life of the photoconductor. From such a viewpoint, a combination of two kinds of fillers was examined, but the problem in which the presence of a large amount of low resistant filler on the surface of photoconductor causes image blur in repeated use cannot be prevented, thus leaving a basic problem unsolved.
To suppress presence of residual potential in the related art, use of photoconductive layer as the protective layer is disclosed (Japanese Patent Publication Nos. 44-834, 43-16198, and 49-10258). However, since the amount of light reaching the photosensitive layer is reduced due to absorption of light by the protective layer, the problem of deterioration in sensitivity of the photoconductor arises, while exhibiting less effect.
It is also disclosed to make the protective layer substantially transparent to suppress accumulation of residual potential by determining average particle diameter of a metal or metal oxide contained in the filler to be 0.3 μm or less (Japanese Patent Application Laid-Open No. 57-30846). In this method, an effect of suppressing an increase in residual potential was confirmed, it is not sufficient to provide basic solution to the problem. An increase in residual potential when filler is contained may possibly be caused by the charge trap or the dispersibility of the filler due to presence of the filler, rather than by the charge generating efficiency. The transparency may be ensured by improving the dispersibility even when the average particle diameter of the filler is 0.3 μm or more, while transparency of the film is sacrificed when the filler is rather coagulated even when an average particle diameter is 0.3 μm or less.
Other means for suppressing the rise in residual potential include: addition of Lewis acid or the like in the protective layer (Japanese Patent Application Laid-Open No. 53-133444); addition of organic protonic acid in the protective layer (Japanese Patent Application Laid-Open No. 55-157748); addition of electron receiving material (Japanese Patent Application Laid-Open No. 2-4275); and addition of wax having acid value of 5 (mgKOH/g) or less (Japanese Patent Application Laid-Open No. 2000-66434). These methods conceivably are based on observation that charge easily reaches the surface when charge injecting property is improved at the interface of protective layer/charge transporting layer and forming of a low resistant portion in the protective layer. Although the effect of reducing the residual potential is confirmed in these methods, they have a side effect such as image blurring and the like to clearly show in the image. Further, addition of an organic acid tends to cause deterioration in dispersibility of the filler, and its effect is insufficient to solve the problem.
To realize a higher image quality in an electrophotographic photoconductor containing filler to improve durability, it is important not only to suppress the image blurring or rise in residual potential, but also for the charge to linearly reach the surface of the photoconductor without being disturbed by the filler in the protective layer. The dispersibility of the filler in the protective layer film has a great influence on the wear resistance. When the charge injected to the protective layer from the charge transporting layer moves to the surface of the protective layer, the move of the charge may be disturbed by the filler coagulated, thus the dot formed by the toner is dispersed, and consequently deteriorates resolution. When the protective layer is provided, the light transmitting property tends to deteriorate due to scattering of the recording light by the filler. Such phenomenon also has a serious adverse effect on the resolution. The influence on the light transmitting property is also closely related to the dispersibility of the filler. The dispersibility of the filler also has a great influence on the wear resistance. The filler when highly coagulated will affect wear resistance due to poor dispersibility. Accordingly, to achieve high image quality simultaneously with high durability in an electrophotographic photoconductor having a protective layer containing the filler for improvement of durability, it is important not only to suppress the image blurring or rise in residual potential, but also to improve dispersibility of the filler in the protective layer film.
Effective means for solving both of the problems at the same time have not been presented as of today. When the filler is contained in the protective layer of the electrophotographic photoconductor to improve durability, the influence of image blurring or rise in residual potential is caused, leaving the problem of image quality improvement unsolved. Further, improvement in durability of a electrophotographic photoconductor having small diameter which requires highest durability from the standpoint of loading the drum heater in order to reduce such influence has not been realized, thus making downsizing of the device, and reduction of power consumption, difficult.
Organic photoconductors which has been surpassing inorganic photoconductors in terms of photosensitivity, spectral sensitivity range, non-pollution property, electrostatic durability and the like, the improvement in mechanical durability is a pressing need to fully utilize their advantages, and the development of such organic photoconductors having improved mechanical strength has been desired for use in highly durable machines and process designs.
When the life of the photoconductor is free from image scraping as the result of improvement in wear resistance of the photoconductor, the life of the photoconductor depends on the electrostatic life of the electrophotographic photoconductor. Concretely, the reduction in electrostatic property of the electrophotographic photoconductor (particularly, local potential leak) causes a defect appearing as spots (toner deposition on the background of images, black spot, etc.) in a surface portion (white) which is not present on a document to be copied. Such defect is often mistaken as a dot in a drawing or a period, comma or the like in an English document, and may be a fatal defect of image.
As described above, the toner and photoconductor used for an image forming method aiming at high transfer ability are required to have excellent releasability. In Japanese Patent Application Laid-Open No. 11-272003 or the like, an electrophotographic photoconductor characterized by having a large contact angle of the outermost layer surface of the electrophotographic photoconductor with pure water is proposed. However, even if the contact angle of the outermost layer surface of the electrophotographic photoconductor with pure water is increased, there is no correlation of releasability of the toner to the electrophotographic photoconductor in actual situations, further, such electrophotographic photoconductor has insufficient transfer ability and cleanability and requires a further improvement.